For many industrial and scientific light sources, harmonic generation of the fundamental wavelength is required. This is typically achieved through the combined use of crystal materials such as LBO, BBO and CLBO, which efficiently convert infrared light beams with an optical frequency, ω, into light beams with optical frequencies of 2ω, 3ω, 4ω and higher harmonics. See U.S. Pat. No. 5,943,351 (Zhou); U.S. Pat. No. 6,061,370 (Yin).
Often, the end user desires the output to be free of light at frequencies other than the highest harmonic generated by the device. This requirement constrains the design by the necessary addition of dichroic mirrors or dispersive prisms for spatially separating the beams of unwanted wavelengths. In the former case, the optical coatings required can be complicated and costly. The coatings and mirror substrates are subject to damage, especially for high intensity or high pulse energy ultraviolet beams (UV). In the later case, optical coatings can be avoided, however the prisms can still exhibit bulk damage if the beam size is too small, and the propagation distance required to separate the wavelengths can be a significant constraint to the device design.
Other optical elements in the laser cavity can indirectly experience damage due to contamination by outgassing processes in the presence of UV radiation.
In many UV lasers, the output from the UV generating harmonic crystal is often directed upon wavelength separation optics in close proximity to the exit port, in an effort to minimize the number of components susceptible to damage. This can result in a beam size that is too small which can cause optical damage to these components.
Prior art addresses the minimization of the contamination of the optical components by purging of the entire cavity in a closed loop. U.S. Pat. No. 7,239,656. However, this approach requires an extended footprint of the laser to be purged, which can often be costly, as well as detrimental to the stability of the cavity by introducing unnecessary air flow currents. It is well known that mode-locked solid state oscillators are particularly sensitive to air currents.
Other approaches use a small separate enclosure to contain the harmonic crystals and the wavelength separation optical components. This, in turn, restricts these optical components to be near the harmonic crystals, where the small size of the beams increases the risk of damage to components.